Effects of cytosolic Ca2+ on the Ca2+ content of the sarcoplasmic reticulum in saponin-permeabilized rat ventricular trabeculae

 Rat ventricular trabeculae were mounted for isometric tension recording, and then permeabilized with saponin. The Ca²⁺ concentration (Ca²⁺]) within the permeabilized preparation (cytosolic Ca²⁺]) was monitored continuously using Indo-1 and the integrals of Ca²⁺ transients resulting from brief caffeine application used as an index of the sarcoplasmic reticulum (SR) Ca²⁺ content. The relationship between SR Ca²⁺ content and cytosolic Ca²⁺] was studied within the reported physiological range (i.e. 50–250 nmol · l^–1 Ca²⁺). Increasing cytosolic Ca²⁺] from 50 nmol · l^–1 to 250 nmol · l^–1 increased the steady-state SR Ca²⁺ content about threefold. However, increasing Ca²⁺] above 250 nmol · l^–1 typically resulted in spontaneous SR Ca²⁺ release, with no further increase in SR Ca²⁺ content. The SR Ca²⁺ content increased only slowly when cytosolic Ca²⁺] was increased; it was unchanged 20 s after a rapid increase in cytosolic Ca²⁺], but increased progressively to a new steady-state level during the following 1–2 min. In a parallel series of experiments using intact papillary muscles, increasing extracellular Ca²⁺] (from 0.5 to 5 mmol · l^–1) significantly increased twitch tension within 20 s of the solution change. These results support previous suggestions that the SR Ca²⁺ content may increase when diastolic cytosolic Ca²⁺] rises during inotropic interventions such as increased stimulus rate or extracellular Ca²⁺]. However, the rate at which SR Ca²⁺ responds to changes in cytoplasmic Ca²⁺] within the diastolic range does not appear rapid enough to explain the early potentiation of twitch tension in intact preparations after an increase in extracellular Ca²⁺]. Received: 26 August 1997 / Accepted: 28 October 1997